Until the 19th Century, each country tended to keep its own zero meridian. Up to the 17th Century, one could imagine the chaos of such a world. One country's longitude was inapplicable to all other countries.
On June 22, 1675 by Charles II of England established the Royal Observatory. It was established to locate celestial bodies more accurately and thereby improve navigation at sea. The observatory was placed in Greenwich (now a borough of London). The principal problem of navigation at the time was the inability to find longitude - one's exact position east and west - while at sea and out of sight of land. It was impossible to calculate it at sea. John Flamsteed was the first Astronomer Royal in March 1675. He began his observations in 1676.
Until 1971, the director of the Royal Greenwich Observatory held the title of Astronomer Royal. One director, Edmond Halley, was the first person to predict successfully the return of a comet-the reappearance in 1758 of the comet that now bears his name.
The Time Ball was erected in 1833 was one of the world's first visual time signals. It dropped daily at 1300h and was used to check marine chronometers by sailors on the Thames.
In 1884, an international conference decided that the meridian, which passed through the Royal Greenwich Observatory, would be the world's prime meridian. 25 countries were represented. The prime meridian was designated 0" longitude, and all other meridians of longitude are numbered east or west of it.
The observatory had played a key role in early navigation and in the development of timekeeping methods needed for navigation. The Greenwich Meridian is also the starting point for the world's time zones. There are 24 time zones, each with a width of 15" longitude. The Greenwich Meridian lies in the middle of a time zone. Moving east of Greenwich, the time becomes one hour later with each time zone entered. Moving to the west, the time becomes one hour earlier with each zone
From 1948 to 1957, the Royal Greenwich Observatory was moved to Herstmonceux Castle, near Eastbourne. In 1960, Flamsteed House was transferred to the care of the National Maritime Museum. It was moved to Cambridge in 1990 and closed in 1998. But the original Greenwich site is still the location of the prime meridian. The Greenwich meridian passes through the original observatory site, where a science museum opened in 1993.